Cholesteric liquid crystal layer, optical element, lighting device, and liquid crystal display

Abstract

A cholesteric liquid crystal layer which is inhibited from appearing to be colored when viewed from slant directions and thus has excellent viewing angle characteristics, while enabling an improvement in the brightness of liquid crystal displays, etc.; and an optical element and an lighting device with each of which liquid crystal displays and the like excellent in brightness and perceptibility can be fabricated. The cholesteric liquid crystal layer (1) has a helical pitch changing in the thickness direction and shows circular dichroism over a wavelength region having a width of at least 50 nm and including a wavelength of 550 nm, wherein the linearly polarized light obtained through a quarter-wavelength plate has a maximum degree of polarization at a wavelength not shorter than 550 nm, or the degree of polarization thereof in a wavelength region on the longer-wavelength side of 550 nm is higher than that in a wavelength region on the shorter-wavelength side of 550 nm. The optical element comprises a multilayer structure composed of the cholesteric liquid crystal layer and at least one optical layer superposed thereon which is a retardation compensating plate, a polarizing plate, a light conductive plate, or a quarter-wavelength plate having an Nz of -1.2 or larger and disposed on the longer-helical-pitch side of the liquid crystal layer. The lighting device or liquid crystal display comprises the cholesteric liquid crystal layer or optical element disposed on one side of a surface light source or liquid crystal cell.

What is claimed is: 
     
       1. A cholesteric liquid crystal layer having Grandjean orientation in which a helical pitch changes in the thickness direction, wherein circular dichroism, by which light is separated into reflected circularly polarized light and transmitted light, is exhibited over a wavelength region having a width of at least 50 nm and including a wavelength of 550 nm, said cholesteric liquid crystal layer showing such polarization degree of the reflected or transmitted light that either, (1) the polarized light in the said wavelength region has a maximum value of the degree of polarization at a wavelength of 550 mn or longer or (2) the degree of polarization of the light integrated over wavelength region where the circular dichroism is exhibited, is higher on the longer-wavelength side of 550 nm than on the shorter-wavelength side of 550 nm. 
     
     
       2. The cholesteric liquid crystal layer of claim 1, wherein the wavelength region where the circular dichroism is exhibited include a visible light region of from 430 to 650 nm, and the linearly polarized light obtained through a quarter-wavelength plate has, in a monochromatic or longer-wavelength visible region of from 550 nm to a longer wavelength, a degree of polarization higher by from 2 to 40% than in a monochromatic or shorter-wavelength visible region of up to 550 nm, excluding 550 nm. 
     
     
       3. An optical element comprising a multilayer structure comprising the cholesteric liquid crystal layer of claim 1 or 2 and at least one optical layer superposed thereon, said optical layer being a retardation compensating plate, a polarizing plate, a light conductive plate, or a quarter-wavelength plate which comprises one or more retardation layers including a retardation layer having a value of Nz defined by the equation (n x  -n z )/(n x  -n y )=Nz of -1.2 to 0.2, wherein n x  is the maximum in-plane refractive index, n y  is the refractive index in a direction perpendicular to the n x  direction, and n z  is the thickness-direction refractive index, provided that when the optical layer is the quarter-wavelength plate, it is disposed on the longer-helical pitch side of the cholesteric liquid crystal layer. 
     
     
       4. The optical element of claim 3, wherein the quarter-wavelength plate comprises at least one retardation layer in which Nz is from -0.2 to -0.8. 
     
     
       5. A lighting device comprising a surface light source and disposed thereover the cholesteric liquid crystal layer of claim 1 or 2. 
     
     
       6. A liquid crystal display comprising a liquid crystal cell and disposed on one side thereof the cholesteric liquid crystal layer of claim 1 or 2. 
     
     
       7. A liquid crystal display comprising the lighting device of claim 5 and a liquid crystal cell disposed on the light emission side thereof. 
     
     
       8. A lighting device comprising a surface light source and disposed thereover the optical element of claim 3. 
     
     
       9. A lighting device comprising a surface light source and disposed thereover the optical element of claim 4. 
     
     
       10. A liquid crystal display comprising a liquid crystal cell and disposed on one side thereof the optical element of claim 3. 
     
     
       11. A liquid crystal display comprising a liquid crystal cell and disposed on one side thereof the optical element of claim 4. 
     
     
       12. A liquid crystal display comprising the lighting device of claim 8 and a liquid crystal cell disposed on the light emission side thereof. 
     
     
       13. A liquid crystal display comprising the lighting device of claim 9 and a liquid crystal cell disposed on the light emission side thereof.